This invention relates to a time base error correcting apparatus for use in an information reproducing apparatus such as a video disk player.
FIG. 1 is a block diagram showing the structure of a prior art time base error correcting apparatus used in a video disk player. Referring to FIG. 1, an information signal recorded on a recording medium or disk 1 is picked up or detected by an information pickup means 2 including a signal detector and a pre-amplifier to provide a detected information output signal, and this output signal is demodulated in an FM demodulation stage 3 including an FM demodulating circuit to provide a demodulated information output signal 4. The frequency of the chrominance subcarrier in this output signal 4 from the FM demodulation stage 3 is lower than the normal frequency since the chrominance subcarrier frequency has been shifted to a lower level during recording the information signal on the disk 1. In a frequency conversion stage 5, the chrominance subcarrier frequency having been shifted to the lower level is converted into the normal frequency. In a burst gate stage 6, the burst signal is extracted from the chrominance information on the chrominance subcarrier converted into the normal frequency. In a phase comparison (phase detector) stage 7, the phase of the extracted burst signal is compared with that of a reference frequency signal applied from a reference frequency oscillator 8, and an output signal 9 indicative of the result of phase comparison appears from the phase comparison stage 7. This output signal 9 from the phase comparison stage 7 is applied to a sample-hold stage 10 to be subjected to sampling-holding and is then applied to a voltage controlled oscillator 12 (abbreviated hereinafter as a VCO) through a filter stage 11 including a lag-lead filter and a DC amplifier to be utilized for the control of the VCO 12. The output signal from the VCO 12 is applied to another input terminal of the frequency conversion stage 5 so that the aforementioned frequency conversion can be satisfactorily carried out.
The output signal from the sample-hold stage 10 is applied also to the information pickup means 2 through a pickup control unit 13 including a phase compensating stage and a driver stage to be utilized for the control of the position of the signal detector of the information pickup means 2 relative to the disk 1. The output signal 4 from the FM demodulation stage 3 is also applied to a synchronizing signal separating circuit 14 in which the synchronizing signal is separated from the input signal 4. The synchronizing signal thus separated is applied to a burst gate pulse generating circuit 15 which generates the burst gate pulses used in the burst gate stage 6 and the sampling pulses used in the sample-hold stage 10. The video output signal appears on an output terminal 20 to be applied to a video reproducing system.
The time base error correcting apparatus having such a construction has been generally used to correct an error of the time base of the luminance and chrominance loop by the system including the pickup means 2 and also to correct an error of the time base of the chrominance loop by the system including the VCO 12.
In the prior art apparatus shown in FIG. 1, a pulse voltage as shown in FIG. 2(a) has tended to appear in the output signal from the sample-hold stage 10 immediately after the period of time Tb (=9 H) in which the burst signal disappears in the vertical blanking period, and a transient response, as shown in FIG. 2(b), resulting in improper color reproduction or skewing in an upper portion of a reproduced picture has tended to appear in the output signal from the pickup control unit 13 due to the appearance of the pulse voltage shown in FIG. 2(a).
Such a pulse voltage appears generally due to the fact that the phase of the chrominance signal changes stepwise in the period immediately after the period Tb since no burst signal is present in this period Tb and a large variation of the time base (phase variation) occurs in the level of the chrominance frequency in this period Tb. Therefore, the prior art apparatus has been defective in that the system design becomes quite complex and difficulty is encountered for increasing the gain of the system including the information pickup means 2 beyond a limit in order that the improper color reproduction or skewing described above can be suppressed or reduced to a practically allowable leave.